'A' Level Medical Option Questions - The Endoscope

Q2. The diagram shows a glass optical fibre with a central core of refractive index 1.55 and a surrounding cladding of refractive index 1.40.

(a) Calculate the critical angle, C, for the boundary between these two types of glass.

sin i/ sin r = n

sin 1/sin 2= sin C/ sin 90° = n2/n1

sin C = 1.40/1.55 = 0.903

angle C = 64.6°

(3 marks)

(b) Complete the path of the light ray shown in the diagram.

• reflection must be on only on the outer edge of core
• two to four reflections drawn
[no marks at all for a zig-zag that gives no thought to angle of incidence = angle of reflection]

Note that it is a good idea to make the ray bend the right way on leaving the fibre too. For this question there was no mark for that - but some have expected such detail in the past!

(2 marks)

(c) State and explain whether the following changes in the optical fibre would increase or decrease the probability of light escaping from the fibre.

(i) increasing the refractive index of the cladding

A smaller difference between the core index and cladding index makes the critical angle larger therefore increases the chance of light escaping

(ii) bending the fibre into a tighter curve

This makes the internal angle of incidence at core-cladding interface more likely to be less than the critical angle
therefore increases the chance of light escaping

(3 marks maximum)

(Total 8 marks)